Control circuit for single stroke electrical tools

ABSTRACT

A simplified control circuit for a single stroke electrical tool comprises a first triggerable gate connected in series with the tool across a source of supply. A series circuit including a second gate, a trigger capacitor, and a manually actuated switch is then connected between a trigger terminal and a current terminal of the first gate. A silicon controlled rectifier pulse generating circuit supplies synchronizing pulses of one polarity to the second gate and charging current of the opposite polarity to the trigger capacitor. Positive feedback from the trigger terminal of the first gate to the input of the second gate gives improved triggering sensitivity and renders the circuit relatively insensitive to changes in the value of components. Multiple firing is prevented by connecting a resistor or diode between the trigger capacitor circuitry and the trigger terminal of the first gate, and by utilizing the voltage drop across the load to aid in fully discharging the trigger capacitor.

United States Patent Naber CONTROL CIRCUIT FOR SINGLE STROKE ELECTRICALTOOLS Inventor: Joseph S. Naber, Marengo, Ill.

Assignee: Fastener Corporation, Franklin Park, Ill.

Filed: June 16, 1969 Appl. No.: 833,321

[ 1 May 9, 1972 [5 7] ABSTRACT A simplified control circuit for a singlestroke electrical tool comprises a first triggerable gate connected inseries with the tool across a source of supply. A series circuitincluding a second gate, a trigger capacitor, and a manually actuatedswitch is then connected between a trigger terminal and a currentterminal of the first gate A silicon controlled rectifier pulsegenerating circuit supplies synchronizing pulses of one polarity to thesecond gate and charging current of the opposite polarity to the triggercapacitor. Positive feedback from the trigger terminal of the first gateto the input of the second gate gives improved triggering sensitivityand renders the circuit relatively insensitive to changes in the valueof components. Multiple firing is prevented by connecting a resistor ordiode between the trigger capacitor circuitry and the trigger terminalof the first gate, and by utilizing the voltage drop across the load toaid in fully discharging the trigger capacitor.

20 Claims, 3 Drawing Figures PATENTEDMAY 9 1972 3,682,196

FIG. 2

38 T FIRST H8 2 GATE VAC. 54 so Hz 2' SECOND M 20 4 0 GATE 1 PULSEGENERATOR VAC 60H:

INVliNI'UR. JOSEPH S. NABER CONTROL CIRCUIT FOR SINGLE STROKE ELECTRICALTOOLS This invention relates to a circuit for controlling theenergization of a load and, more particularly, to such a circuit forcontrolling an electrically operated fastener driving tool.

U.S. Pat. No. 3,179,866, assigned to the same assignee as the presentapplication, discloses an electrically operated fastener driving toolthat is completely self-contained in the sense that all of the drivingcomponents and the power supply therefor are carried on the housing ofthe tool. This tool includes a fastener driving element or bladeactuated by a winding means that is selectively connected to analternating current potential source for no more than one cycle of thepotential by a manually actuated power supply or control circuit carriedon the housing. During the momentary energization of the winding, thefastener driving blade is driven through a single power stroke duringwhich the fastener or staple is driven, and the tool cannot bereoperated through an additional power stroke until the manuallyactuated control means is released and reoperated. This tool is capableof continuous use without excessive heating, provides adequate drivingpower and is easily controlled in requiring the actuation of only asingle switching means.

Control circuits of the type disclosed in the above-mentioned patentgenerally utilize a trigger capacitor to trigger a silicon contr )lledrectifier. This capacitor is discharged quickly to prevent a secondfiring. Occasionally, however, circuits of this t pe allow a second, andeven a third firing to occur. This not only can overload the source ofpower, but it also can be dangerous to the operator, especially in amachine that automatically reloads itself between firings.

Such circuits also tend to be somewhat bulky, mainly because theyusually include in their circuitry electrolytic capacitors havingcapacity values of microfarads or greater, and oftentimes havingcapacity values in excess of 100 microfarads. These capacitors are notonly expensive and physically large in size, but they also are subjectto deterioration with time, and are a frequent cause of malfunction.

A primary object of the present invention is therefore the production ofa control circuit that is less expensive, more compact, and morereliable than control circuits currently in use.

A more specific object of the present invention is the production of acontrol circuit in which a margin of safety is provided against multiplefirings.

A further object of the present invention is the production of a controlcircuit that does not include any large capacity capacitors.

Briefly stated, a control circuit designed in accordance with thepresent invention comprises a first triggerable gate connected in serieswith a solenoid or load across a source of alternating currentpotential, and a trigger capacitor that is connected in series with amanual switch and a second synchronized gate between a control input anda current terminal of the first triggerable gate. A diode or resistorelement is connected in series with the gate control input to preventtriggering of the gate before the trigger capacitor applies asubstantial voltage to the resistor or diode element. A second resistorserially connects the resistor or diode element to the opposite side ofthe load, thereby applying a reverse bias to the gate control inputunder standby conditions and insuring that the trigger capacitor isfully discharged after each operation of the circuit. A feedbackresistor connects the first triggerable gate control input to the inputof the second synchronized gate. This resistor applies additionalreverse bias current to the gate control input, improves triggeringsensitivity, and renders the circuit relatively insensitive to changesin the values of components. To prevent the gate control input frombeing biased in a reverse direction beyond its breakdown potential, andalso to further suppress any tendency of the circuit to trigger before asubstantial potential is applied to the gate control input, anotherresistor is connected from the gate control input to the adjacentcurrent terminal of the first triggerable gate. All of the above-circuitfeatures combine to produce a circuit which is highly unlikely ever tofire a second time.

The synchronized gate is enabled at the start of every positive halfcycle by trigger pulses generated by a pulse generator. The pulsegenerator comprises a silicon controlled rectifier connected in serieswith a resistor across a suitable source of potential. When thepotential across this network goes positive, a voltage pulse momentarilyappears across the controlled rectifier and then is terminated when thedevice becomes conductive. The gate of the controlled rectifier isbiased so that this pulse only appears during the initial or leadingportion of each positive half cycle. These pulses are fed into thesecond synchronized gate. This arrangement insures that the triggerablegate always connects the load to the supply voltage for close to acomplete half cycle, rather than for just a portion of the half cycle,and prevents the trigger capacitor from discharging during negative halfcycles. During negative half cycles the controlled rectifier remainsnonconductive, and therefore can be used as a convenient source ofnegative current for recharging the trigger capacitor.

Preferably the manual switch is a single pole, single throw switchconnecting one end of the trigger capacitor to a current terminal of thefirst triggerable gate or to one of the supply terminals. Chargingcurrent for the trigger capacitor can then flow through a resistorconnected between this same end of the trigger capacitor and the pulsegenerator. Under certain circumstances, however, a single pole doublethrow switch must be used to alternately switch the trigger capacitorbetween the source of charging current and the current or supplyterminal.

The only capacitor in the control circuit is the trigger capacitor,which need only be large enough to supply a trigger current pulse to thetriggerable gate. Typically, this capacitor will have a value of 0.1 or0.2 microfarads. Thus, electrolytic capacitors are not required, and themaintenance, cost, and packaging problems created by electrolyticcapacitors cannot arise. The resulting circuit is simple, reliable, andinexpensive. After each firing of the triggerable gate, the triggercapacitor is fully discharged. Since a trigger capacitor charge inexcess of ten volts would be required to trigger the circuit a secondtime, a safety margin is provided against possible multiple firingswhich might otherwise be caused by noise pulses, improper tolerances, orfaulty switch contacts.

Further objects, features, and advantages of the present invention willbecome apparent as the following description proceeds, and the featuresof novelty which characterize the invention will be pointed out withparticularity in the claims annexed to and forming a part of thisspecification.

For a better understanding of the present invention, reference may behad to the accompanying drawings in which:

FIG. 1 is a simplified block diagram of a controller circuit designed inaccordance with the present invention;

FIG. 2 is a block diagram of a controller circuit designed in accordancewith the present invention and including synchronization means; and

FIG. 3 is a complete schematic diagram of a controller circuit designedin accordance with the present invention.

Referring now to FIG. 1, there is shown a simplified block diagram of acontrol circuit designed in accordance with the present invention andindicated by the reference numeral 10. The control circuit 10 isenergized by a source of a volt AC, 60 HZ current which is supplied totwo supply nodes 12 and 14. Connected between the supply nodes 12 and 14is a series circuit comprising a first triggerable gate 18 connected inseries with a solenoid or load 16. The node 20 common to both of theseelements is called the common node. A trigger node 21 is connected to agate or control terminal 38 of the first gate 18 by a diode or resistor26 (if a diode is used, its anode is adjacent the trigger node 21, asshown with dashed lines in FIG. 3). The trigger node 21 is alsoconnected to the supply node 14 by a resistor 34. The two nodes 20 and21 are connected together by a series circuit that includes a triggercapacitor 54 and a switch 56.

The gate 18 is assumed to be a silicon controlled rectifier, a siliconcontrolled switch, a thyratron, or some equivalent form of triggerablegating device or circuit. While triggerable gates of this type come in avariety of types and polarities, it will be assumed for purposes ofdiscussion that the gate 18 is of a type such that a positive currentsupplied to the control terminal 38 causes the gate 18 to conduct, butonly when the supply node 12 is positive with respect to the common node20.

Initially the switch S6 is open, and the first gate 18 is nonconducting.No current flows through the load 16 and negligible current flowsthrough the resistors 34 and the resistor or diode 26, and the controlterminal 38 is at approximately the same potential as the supply node14. No current flows into the control terminal 38. The use of a diodefor element 26 further resists current flow into the control terminal38, since any such current has to overcome the junction potential of thediode before it can flow into the control terminal 38. Thus, the gate 18remains non-conductive. The trigger capacitor 54 is assumed to beinitially charged by any convenient charging means, so that the end ofthe capacitor 54 connected to the trigger node 21 is charged positivelywith respect to the end of the capacitor 54 connected to the switch 56.lf the switch 56 is now closed at the start of a half cycle when thesupply node 12 is positive with respect to the supply node 14, apositive current flows through the loop comprising the resistor or diode26, the control terminal 38, the gate 18, the switch 56, and thecapacitor 54. This current triggers the first gate 18 and causes it toconnect the common node 20 to the supply node 12. In this manner, apositive half cycle of potential is impressed upon the load 16.

While this positive half cycle is impressed across the load 16, thecapacitor 54 is charged oppositely from its initial charge by currentflowing through the resistor 34. Thus, at the end of the positive halfcycle, the capacitor 54 is left with a reverse negative charge withtends to hold the trigger node 21 negative with respect to the commonnode 20.

During the negative half cycle which follows, the gate 18 ceases toconduct, the flow of current to the load 16 stops, and the negativepotential developed across the capacitor 54 disappears, leaving thecapacitor 54 essentially discharged. When once again the supply node 12goes positive with respect to the supply node 14, the trigger capacitor54 cannot supply any further current to the control terminal 38. Sinceit takes a substantial potential across the capacitor 54 to force enoughcurrent through the resistor or diode 26, a potential margin againstmultiple firing of the gate 18 is provided.

FIG. 2 shows the control circuit in more detail, and in particularincludes those elements of the circuit 10 which synchronize thedischarge of the trigger capacitor 54 with leading edge of a positivehalf cycle of the supply current. In particular, a second gate 22 and apositive half cycle leading edge pulse generator 40 are added to thecircuit 10. The second gate 22 is connected in series with the triggercapacitor 54 and the switch 56. This second gate 22 prevents the triggercapacitor 54 from discharging and supplying current to the controlterminal 38 of the first gate 18 until the leading edge of a positivehalf cycle. The second gate 22 is enabled only when the pulse generator40 is generating a positive pulse. The pulse generator 40 is arranged togenerate a narrow positive pulse whenever the supply node 12 goespositive with respect to the supply node 14. The details of the pulsegenerator 40 are disclosed below in the discussion concerning FIG. 3.The switch 56 may be thrown at any time, and does not need to be thrownat the start of a positive half cycle. The second gate 22 prevents aclosure of the switch 56 from having any efiect until the proper time.

Referring now to FIG. 3, there is shown a complete schematic diagram ofthe control circuit 10. The first gate 18 is a conventional siliconcontrolled rectifier. The load 16 is shown as an inductance or solenoid.This would typically be the case, since most tools of this type aresolenoid actuated. The second gate 22 comprises a transistor 24 havingits emitter electrode connected to the trigger node 21, its baseelectrode connected to the pulse generator 40, and its collectorelectrode 36 connected to the trigger capacitor 54. A single pole doublethrow (SPDT) switch 57 replaces the single pole single throw (SPST)switch 56 of FIGS. 1 and 2. As will be explained in more detail below, asingle pole single throw (SPST) switch can also be used to trigger thecircuit.

The pulse generator 40 comprises basically a silicon controlledrectifier 42 connected in series with a resistor 44. This series circuitis connected between the supply node 12 and the common node 20, asshown. The control terminal 50 of the silicon controlled rectifier 42 isconnected to the common node 20 by a resistor 48 and to the supply node12 by a resistor 46. A node 52 is the node common to both the resistor44 and the controlled rectifier 42 and is connected to the second gate22 by a resistor 32.

Assuming for the moment that the triggerable gate 18 is nonconductive,no current flows through the load 16, and the common node 20 is at thesame potential as the supply node 14. The pulse generator 40 can thus bethought of as connected directly between the supply nodes 12 and 14.When the supply node 12 is negative with respect to the supply node 14,the controlled rectifier 42 is reverse biased and does not conduct.Since the node 52 is connected to the supply node 12 by the resistor 44,the node 52 goes negative along with the supply node 12. This largenegative potential is used to charge the trigger capacitor 54, as willbe explained below. When the supply node 12 again goes positive withrespect to the supply node 14, initially the controlled rectifier 42remains unconductive, and the node 52 goes positive along with the node12. This positive voltage at the node 52 causes a current to flowthrough the resistor 32 and into the gate 22. This current actuates thegate 22 by causing a transistor 24 to conduct. This current is allowedto flow for only a brief time, however, because soon the potential ofthe node 12 goes far enough positive so that current through theresistor 46 flows into the control terminal 50 and triggers thecontrolled rectifier 42 into conduction. This effectively connects thenode 52 to the node 20 and thus cuts off the flow of current through theresistor 32. The values of the resistors 46 and 48 are chosen to causeconduction in the controlled rectifier 42 at the proper time. Thus, apositive potential appears at the node 52 and enables the gate 22 foronly a short period immediately following the leading edge of a positivehalf cycle of supply current.

The trigger capacitor 54 is recharged whenever the switch 57 is in theposition shown in H6. 3. During negative half cycles when the supplynode 14 is positive with respect to supply node 12, current flows fromthe supply node 14, through the load 16, the resistors 28 and 30, andthe base-collector junction of the transistor 24 to the triggercapacitor 54. Simultaneously current flows out of the trigger capacitor54, through the switch 57, a resistor 58, and the resistor 44 to thesupply node 12. This current charges the trigger capacitor 54 and leavesit with a large negative charge.

As mentioned above, a single pole single throw switch can be used inplace of a single pole double throw switch 57, if desired. The switch isconnected between the capacitor 54 and the node 20, as shown in FIG. 2.The resistor 58 (FIG. 3) is then connected directly to the end of thecapacitor 54 adjacent the switch. When the switch is closed to initiatea discharge, the switch shorts the resistor 58 to the common node 20 andthus prevents the capacitor 54 from recharging until the switch isopened. The single pole single throw configuration gives entirelysatisfactory results. However, when the invention is used in a tool thatrecoils, it has been found that certain types of switches have contactswhich open when the tool recoils, thereby permitting the capacitor 54 torecharge prematurely. With such a recoil sensitive switch the singlepole double throw configuration is preferable, since it gives moreprotection against multiple firing.

The resistors 30 and 32 form a voltage divider which couples thepositive pulses from the node 52 to the base of the transistor 24. Byconnecting the resistor 30 to the control terminal 38 rather than to thecommon node 20, positive feedback is achieved which makes the selectionof values for the resistor 34 and for the resistor or diode 26 much lesscritical. When the gate 18 is nonconductive, the control tenninal 38 canbe considered to be short-circuited to the common node 20 by the lowimpedance resistor 28. However, when the gage 18 is conducting heavily,the control terminal 38 goes positive with respect to the common node20. This positive voltage causes current to flow through the resistor 30and into the base of the transistor 24. This current adds to the currentflowing through the resistor 32 from the pulse generator 40 and thusrepresents positive feedback. The positive feedback is further enhancedby the fact that the emitter of the transistor 24 is driven negativewith respect to the common node 20 by the voltage drop which appearsacross the load 16, as is explained above. If the element 26 is a diode,the emitter of the transistor 24 is effectively decoupled from thetrigger node 38 once the trigger capacitor 54 is discharged and allowscurrent flow through the resistor 34 to further discharge the capacitor54. All of these elements in combination produces a circuit that has ahigh trigger sensitivity, an ability to fully discharge the triggercapacitor 54, and yet a circuit which operates properly even though someof the components may be out of tolerance. The resistors 30 and 32 musthave a high enough resistance so that the positive pulses generated atthe node 52 do not trigger the triggerable gate 18 when it is in itsmost sensitive condition.

The series circuit comprising a resistor 62 and a capacitor 60 isconnected across the gate 18 to prevent plug-in" and other linetransients from causing misfirings due to a sudden rise in potentialacross the triggerable gate 18. The resistor element 28 also aids inproviding some immunity to misfirings due to line transients.

Assuming now that a suitable supply of current is applied to the supplynodes 12 and 14, initially the trigger capacitor 54 charges up in themanner described above so that the end of the capacitor adjacent theswitch 57 is negatively charged. When it is desired to supply power tothe load 16, the switch 57 is thrown into the opposite position fromthat shown in FIG. 3. This connects the negative end of the triggercapacitor 54 to the common node 20. The next time the supply lead 12goes positive with respect to the supply lead 14, a positive pulse isgenerated at the node 52 in the manner described above. This positivepulse causes current to flow through the resistor 32, and this currentcauses the transistor 24 to conduct. The transistor 24 connects thetrigger node 21 to the positive side of the trigger capacitor 54. Apositive current now flows through the resistor or diode 26 and into thecontrol terminal 38 of the triggerable gate 18. The gate 18 now conductsand connects the load 16 directly across the supply nodes 12 and 14 forthe remainder of the positive half cycle.

When the control rectifier 18 is conductive, the voltage which appearsacross the load 16 also appears across the series combination ofresistor 28, resistor or diode 26, and resistor 34. Because of thevoltage drop across resistor 28 and resistor or diode 26, the commonnode 20 is now positive with respect to the trigger node 21. Similarly,since the node 52 is either connected to the supply node 12 by theresistor 44 or else is connected to the common node 20 by the controlledrectifier 42, the node 52 is also positive with respect to the triggernode 21. Current therefore flows through the resistor 32 and keeps thetransistor 24 in a conducting state, and simultaneously current flowsout of the trigger capacitor 54 and into the common node 21. Thiscurrent flow completely discharges the trigger capacitor 54 and mayleave a residual charge that biases the common node 20 positive withrespect to the collector 36 of the gate transistor 22.

At the end of the positive half cycle, the triggerable gate 18 turns offand stops the flow of current to load 16, with the possible exception ofa transient current that flows momentarily through the elements 60 and62. The switch 57 still connects the trigger capacitor 54 to the node20, but now the trigger capacitor 54 is discharged, and it holds thecollector 36 of the transistor 24 at roughly the potential of the commonnode 20. Thus, no current flows from the trigger capacitor 54 into thecontrol terminal 38 of the rectifier 18. Since it would take at leasttwo volts to overcome the junction potentials of the transistor 24, thediode 26, and the triggerable gate 18, a margin of insurance is providedwhich insures that the gate 18 does not fire a second time. Ultimately,when the switch 57 is returned to the position shown in FIG. 3, acharging path is reestablished for the trigger capacitor 54, and thecapacitor 54 is recharged.

The capacitor 54 need be only large enough to hold sufficient charge totrigger the silicon controlled rectifier 18. In the present embodiment,this capacitor has a value of 0.2 microfarads. The capacitor 60 has avalue of 0.1 microfarads. Thus, the circuit 10 is entirely free of anylarge or cumbersome capacitors. The use of a second gate 22 and a pulsegenerator 40 obviates the need for an electrolytic capacitor to maintainthe silicon controlled rectifier 18 in a conducting state duringnegative half cycles (see, for example, U.S. Pat. No. 3,414,738). Thesilicon controlled rectifier pulse generator 40 also supplies a largeenough negative potential to the charging current resistor 58 so thatthe trigger capacitor 54 can be substantially smaller than thosepreviously used see, for example, US. Pat. No. 3,179,866).

The improved operation of the circuit 10 is made possible in part by thepositioning of the first gate 18 with respect to the load 16. Inconventional circuits, the load 16 would always be connected between thefirst gate 18 and-the supply node 12, rather than between the first gate18 and the supply node 14. Such prior art arrangements have no way ofsupplying a reverse current to the trigger capacitor 54 through aresistor such as the resistor 34. The negative current flow through theresistor 34 when the triggerable gate 18 is conducting insures that thetrigger capacitor 54 is always fully discharged before the end of theconduction cycle.

The following components are used in the preferred embodiment ofthecontrol circuit 10:

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A control circuit for energizing a load with half cycle pulses ofcurrent from an alternating current supply, said control circuitcomprising:

a first gate having a first control terminal and having two currentterminals;

a first series circuit including said gate and said load, said firstseries circuit connected across said alternating current supply;

a second series circuit including a trigger capacitor, a

switch, and a second gate having a second control terminal, said secondseries circuit connecting said first control terminal to one of said twocurrent terminals; capacitor charging means connecting said switch andsaid capacitor to said supply through a rectifier for charging saidcapacitor whenever said switch is positioned to break said second seriescircuit; and pulse source means for supplying pulses to said secondcontrol terminal during the initial portions of supply current cycles ofone polarity, wherein said pulse source means comprises a voltagesensitive breakdown device connected in series with a first resistiveelement across the supply and wherein said second control terminal isconnected by suitable circuit means to the junction of said breakdowndevice with said resistive element.

2. A control circuit in accordance with claim 1 wherein the capacitorcharging means comprise a second resistive element connected to thejunction of said breakdown device with said first resistive element andarranged to supply a charging current to said trigger capacitor, andwherein said breakdown device breaks down in one direction only and thusserves as a rectifier for the capacitor charging means.

3. A control circuit in accordance with claim 1 wherein the breakdowndevice is a silicon controlled rectifier having a control terminal, andwherein the breakdown device also includes additional resistanceelements connecting said control terminal to circuit nodes of such apotential that the breakdown device becomes conductive shortly after thestart of alternate half cycles of the supply.

4. A control circuit in accordance with claim 3 wherein the chargingmeans comprise a second resistive element connected to the junction ofsaid breakdown device with said first resistive element and arranged tosupply a charging current to said trigger capacitor, said breakdowndevice serving as a rectifier for the capacitor charging means.

5. A control circuit in accordance with claim 4 wherein the switch is asingle pole single throw switch having two contacts, and wherein thetrigger capacitor and said second resistive element connect to one ofthecontacts.

6. A control circuit in accordance with claim 4 wherein the switch is asingle pole double throw switch having a wiper arm and two contacts,wherein the trigger capacitor is connected to the wiper arm of saidswitch, and wherein said second resistive element connects to one of thetwo contacts of said switch.

7. A control circuit in accordance with claim 1 wherein the secondseries circuit also includes a diode oriented so as to allow the flow ofcurrent to said first control terminal.

8. A control circuit in accordance with claim 1 to which has been addeda positive feedback circuit connecting said first control terminal tosaid second control terminal.

9. A control circuit in accordance with claim 8 wherein said positivefeedback circuit comprises a resistor.

10. A pulse generator for supplying an actuating potential to anelectronic gate during the initial portions of supply current halfcycles of one polarity, said gate closing in response to the actuatingpotential, said pulse generator comprising:

a voltage sensitive breakdown device comprising a silicon controlledrectifier having a control terminal, and having the control terminalconnected to the supply current by resistive elements;

a resistive element;

a series circuit including the breakdown device and the resistiveelement connected across the supply current;

and circuit means connecting the breakdown device to the electronic gatefor transmitting the potential developed across the breakdown device tothe electronic gate.

11. A combination pulse generator and unidirectional current generatorfor generating pulses of one polarity during the initial portions ofsupply current half cycles of one polarity and for generating a currentof the opposite polarity during supply current half cycles of theopposite polarity, comprising:

a source of alternating supply current having first and second supplynodes;

a silicon controlled rectifier having a control terminal and havingfirst and second current terminals, said first current terminal beingconnected to said first supply node;

a resistive element connecting said second current terminal to saidsecond supply node;

bias means for biasing said control terminal so that said siliconcontrolled rectifier becomes conductive shortly after the onset ofsupply current half cycles of one polarity, whereby pulses of thatpolarity are generated at said second current terminal;

and a circuit means connected to said second current terminal forreceiving current during supply current cycles of the opposite polarity.

12. A combination pulse generator and unidirectional current generatorin accordance with claim 11 wherein the circuit means comprises a seriescircuit which includes a resistor and a capacitor, and which connectssaid second current terminal to said first current terminal, whereby thecapacitor may be charged to a potential level which is greater than thepotential magnitude of said pulses.

13. A control circuit for energizing a load with half cycles pulses ofcurrent from an alternating current supply, said control circuitcomprising:

a first gate having a first control terminal and having two currentterminals;

a first series circuit including said gate and said load, said firstseries circuit connected across said alternating current supply;

a second series circuit including a trigger capacitor, a

switch, and a second gate having a second control terminal, said secondseries circuit connecting said first control terminal to saidalternating circuit supply;

capacitor charging means connecting said switch and said capacitor tosaid supply through a rectifier for charging said capacitor wheneversaid switch is positioned to break said second series circuit;

and pulse source means for supplying pulses to said second controlterminal during the initial portions of supply current cycles of onepolarity, wherein the pulse source means comprises a voltage sensitivebreakdown device connected in series with a first resistive elementacross the supply and wherein said second control terminal is connectedby suitable circuit means to the junction of said breakdown device withsaid resistive element.

14. A control circuit in accordance with claim 13 wherein the capacitorcharging means comprise a second resistive element connected to thejunction of said breakdown device with said first resistive element andarranged to supply a charging current to said trigger capacitor, andwherein said breakdown device breaks down in one direction only and thusserves as a rectifier for the capacitor charging means.

15. A control circuit in accordance with claim 13 wherein the breakdowndevice is a silicon controlled rectifier having a control terminal, andwherein the breakdown device also includes additional resistanceelements connecting said control terminal to circuit nodes of such apotential that the breakdown device becomes conductive shortly after thestart of alternate half cycles of the supply.

16. A control circuit in accordance with claim 15 wherein the capacitorcharging means comprise a second resistive element connected to thejunction of said breakdown device with said first resistive element andarranged to supply a charging current to said trigger capacitor, saidbreakdown device serving as a rectifier for the capacitor chargingmeansv 17. A control circuit in accordance with claim 16 wherein theswitch is a single pole single throw switch having two contacts, andwherein the trigger capacitor and said second resistive element connectto one of the contacts.

18. A control circuit in accordance with claim 16 wherein the switch isa single pole double throw switch having a wiper arm and two contacts,wherein the trigger capacitor is connected to the wiper arm of saidswitch, and wherein said second resistive element connects to one of thetwo contacts of said switch.

19. A control circuit in accordance with claim 13 wherein the secondseries circuit also includes a diode oriented so as to allow the flow ofcurrent to said first control terminal.

20. A control circuit in accordance with claim 13 to which has beenadded a positive feedback circuit connecting said first control terminalto said second control terminal.

1. A control circuit for energizing a load with half cycle pulses ofcurrent from an alternating current supply, said control circuitcomprising: a first gate having a first control terminal and having twocurrent terminals; a first series circuit including said gate and saidload, said first series circuit connected across said alternatingcurrent supply; a second series circuit including a trigger capacitor, aswitch, and a second gate having a second control terminal, said secondseries circuit connecting said first control terminal to one of said twocurrent terminals; capacitor charging means connecting said switch andsaid capacitor to said supply through a rectifier for charging saidcapacitor whenever said switch is positioned to break said second seriescircuit; and pulse source means for supplying pulses to said secondcontrol terminal during the initial portions of supply current cycles ofone polarity, wherein said pulse source means comprises a voltagesensitive breakdown device connected in series with a first resistiveelement across the supply and wherein said second control terminal isconnected by suitable circuit means to the junction of said breakdowndevice with said resistive element.
 2. A control circuit in accordancewith claim 1 wherein the capacitor charging means comprise a secondresistive element connecTed to the junction of said breakdown devicewith said first resistive element and arranged to supply a chargingcurrent to said trigger capacitor, and wherein said breakdown devicebreaks down in one direction only and thus serves as a rectifier for thecapacitor charging means.
 3. A control circuit in accordance with claim1 wherein the breakdown device is a silicon controlled rectifier havinga control terminal, and wherein the breakdown device also includesadditional resistance elements connecting said control terminal tocircuit nodes of such a potential that the breakdown device becomesconductive shortly after the start of alternate half cycles of thesupply.
 4. A control circuit in accordance with claim 3 wherein thecharging means comprise a second resistive element connected to thejunction of said breakdown device with said first resistive element andarranged to supply a charging current to said trigger capacitor, saidbreakdown device serving as a rectifier for the capacitor chargingmeans.
 5. A control circuit in accordance with claim 4 wherein theswitch is a single pole single throw switch having two contacts, andwherein the trigger capacitor and said second resistive element connectto one of the contacts.
 6. A control circuit in accordance with claim 4wherein the switch is a single pole double throw switch having a wiperarm and two contacts, wherein the trigger capacitor is connected to thewiper arm of said switch, and wherein said second resistive elementconnects to one of the two contacts of said switch.
 7. A control circuitin accordance with claim 1 wherein the second series circuit alsoincludes a diode oriented so as to allow the flow of current to saidfirst control terminal.
 8. A control circuit in accordance with claim 1to which has been added a positive feedback circuit connecting saidfirst control terminal to said second control terminal.
 9. A controlcircuit in accordance with claim 8 wherein said positive feedbackcircuit comprises a resistor.
 10. A pulse generator for supplying anactuating potential to an electronic gate during the initial portions ofsupply current half cycles of one polarity, said gate closing inresponse to the actuating potential, said pulse generator comprising: avoltage sensitive breakdown device comprising a silicon controlledrectifier having a control terminal, and having the control terminalconnected to the supply current by resistive elements; a resistiveelement; a series circuit including the breakdown device and theresistive element connected across the supply current; and circuit meansconnecting the breakdown device to the electronic gate for transmittingthe potential developed across the breakdown device to the electronicgate.
 11. A combination pulse generator and unidirectional currentgenerator for generating pulses of one polarity during the initialportions of supply current half cycles of one polarity and forgenerating a current of the opposite polarity during supply current halfcycles of the opposite polarity, comprising: a source of alternatingsupply current having first and second supply nodes; a siliconcontrolled rectifier having a control terminal and having first andsecond current terminals, said first current terminal being connected tosaid first supply node; a resistive element connecting said secondcurrent terminal to said second supply node; bias means for biasing saidcontrol terminal so that said silicon controlled rectifier becomesconductive shortly after the onset of supply current half cycles of onepolarity, whereby pulses of that polarity are generated at said secondcurrent terminal; and a circuit means connected to said second currentterminal for receiving current during supply current cycles of theopposite polarity.
 12. A combination pulse generator and unidirectionalcurrent generator in accordance with claim 11 wherein the circuit meanscomprises a series circuit which includes a resistOr and a capacitor,and which connects said second current terminal to said first currentterminal, whereby the capacitor may be charged to a potential levelwhich is greater than the potential magnitude of said pulses.
 13. Acontrol circuit for energizing a load with half cycles pulses of currentfrom an alternating current supply, said control circuit comprising: afirst gate having a first control terminal and having two currentterminals; a first series circuit including said gate and said load,said first series circuit connected across said alternating currentsupply; a second series circuit including a trigger capacitor, a switch,and a second gate having a second control terminal, said second seriescircuit connecting said first control terminal to said alternatingcircuit supply; capacitor charging means connecting said switch and saidcapacitor to said supply through a rectifier for charging said capacitorwhenever said switch is positioned to break said second series circuit;and pulse source means for supplying pulses to said second controlterminal during the initial portions of supply current cycles of onepolarity, wherein the pulse source means comprises a voltage sensitivebreakdown device connected in series with a first resistive elementacross the supply and wherein said second control terminal is connectedby suitable circuit means to the junction of said breakdown device withsaid resistive element.
 14. A control circuit in accordance with claim13 wherein the capacitor charging means comprise a second resistiveelement connected to the junction of said breakdown device with saidfirst resistive element and arranged to supply a charging current tosaid trigger capacitor, and wherein said breakdown device breaks down inone direction only and thus serves as a rectifier for the capacitorcharging means.
 15. A control circuit in accordance with claim 13wherein the breakdown device is a silicon controlled rectifier having acontrol terminal, and wherein the breakdown device also includesadditional resistance elements connecting said control terminal tocircuit nodes of such a potential that the breakdown device becomesconductive shortly after the start of alternate half cycles of thesupply.
 16. A control circuit in accordance with claim 15 wherein thecapacitor charging means comprise a second resistive element connectedto the junction of said breakdown device with said first resistiveelement and arranged to supply a charging current to said triggercapacitor, said breakdown device serving as a rectifier for thecapacitor charging means.
 17. A control circuit in accordance with claim16 wherein the switch is a single pole single throw switch having twocontacts, and wherein the trigger capacitor and said second resistiveelement connect to one of the contacts.
 18. A control circuit inaccordance with claim 16 wherein the switch is a single pole doublethrow switch having a wiper arm and two contacts, wherein the triggercapacitor is connected to the wiper arm of said switch, and wherein saidsecond resistive element connects to one of the two contacts of saidswitch.
 19. A control circuit in accordance with claim 13 wherein thesecond series circuit also includes a diode oriented so as to allow theflow of current to said first control terminal.
 20. A control circuit inaccordance with claim 13 to which has been added a positive feedbackcircuit connecting said first control terminal to said second controlterminal.